This invention relates to a stop switch apparatus for an engine in which an engine stop switch unit is integrated with an engine stop switch circuit. More particularly, the invention concerns a stop switch apparatus for an engine in which a source circuit from a magneto is earthed to forcibly stop the engine.
Heretofore, regarding a stop switch apparatus for an engine of this type, as disclosed, for example, in Japanese Patent Laid-Open Publication No. 18668/1991 or Japanese Utility Model Laid-Open Publication No. 30381/1982, in most stop switch apparatuses, when a stop switch for the engine is closed, a thyristor for stopping the engine of an engine stop switch circuit is turned on to charge a holding capacitor. Then, a turn-on voltage is applied to a gate of the thyristor for stopping the engine by a discharge voltage from the holding capacitor at a certain time constant. Even if the engine stop switch is returned to an off state, a cut-off of a primary side of an ignition coil is disabled by turning on the thyristor for stopping the engine, thereby to misfire the engine to stop the engine.
A normally open switch having a metal contact is frequently used as the engine stop switch.
In the above-described prior art, structures of the engine stop circuit and the engine stop switch are not clearly disclosed, but it is general to separately provide the engine stop circuit and the engine stop switch.
Accordingly, the engine stop switch apparatus is relatively increased in shape, and assembling is complicated due to necessity of wiring the engine stop circuit to the engine stop switch. In addition, the number of components is increased, and hence there is a problem in reliability of a product.
Since the metal contact is used in the switch, a contact defect easily occurs due to generation of rust, and there is a problem in durability in the case of use outside of a housing for a long period of time.
Heretofore, regarding a general-purpose engine having a magneto as a power supply, when an engine is stopped, a source circuit voltage applied to a primary side of an ignition coil is short-circuited by a thyristor circuit to eliminate induction of an insulating breakdown voltage at a secondary side of the ignition coil, thereby forcibly stopping the engine.
In general, a thyristor in the thyristor circuit is turned on by a signal from an engine stop switch or a control circuit for outputting an engine stop detection signal if an oil level becomes lower than a reference level, and a short-circuiting current of a magneto power supply is supplied to the thyristor through a short-circuiting resistor connected in series with the thyristor. For example, prior art is disclosed in Japanese Patent Laid-Open Publication No. 18668/1991 filed by the same assignee as this patent application. However, when the thyristor is turned on to short-circuit the magneto power supply, a relatively large peak voltage generated at both ends of the short-circuiting resistor is amplified by the ignition coil due to an irregularity in a voltage across the short-circuiting resistor caused by an irregularity in the magnetos and further superposition of high frequency components in the case of the short-circuit on the voltage at both ends of the short-circuiting resistor, and applied to the ignition plug. Then, there is a fear of occurrence of an inductive discharge due to application of the irregularity in an ignition system as well.
Therefore, even if the thyristor is turned on to short-circuit the magneto power supply, the engine is not easily stopped, and afterburn occurs in the engine, thereby decreasing durability of the engine.